High Processor Count Kills Caustics Speed

[Joelaff]

Anyone else notice that FEWER processors seems to calculate caustics MUCH faster than machines with a lot of procs (like 32-96 procs)?

Is there anything that can be done about this to limit the number of CPUs specifically for caustics? Thanks.

Note: It would be EXCEEDINGLY helpful to have something like VRAY_NUM_THREADS that applies only to caustics photon mapping calculations. 96 core machines are reduced to slower than a 7950X without it.

[vladimir_krastev]

Hi, thanks for posting.

Anyone else notice that FEWER processors seems to calculate caustics MUCH faster

Are you using progressive caustics? If you are that is a limitation that the progressive caustics may have in some scenes.
From the docs:

• Depending on the scene, the performance might not scale linearly with the number of threads/cores, resulting in inefficient CPU utilization.

[Joelaff]

Thanks for the reply, Vladamir.

Actually, this is Photon mapped caustics. I was avoid progressive because it says you should use a time limit, and we have a heterogeneous farm; so it is not possible to set a time limit that would be consistent during an animation rendered with different types of nodes.

And what I am seeing is that caustics photon calculation in scaling INVERSELY proportional to the number of cores. More cores == slower performance than few cores.

[Muhammed_Hamed]

Hi Joelaff

What CPUs are you using?
I'm comparing the 64-core 3990X and 7950X, the scaling between the 2 processors seems to be ok on my side. What are your Caustics settings and sampler settings?

Best,
Muhammed

[Joelaff]

Comparing a 7950X with AMD EPYC 9R14 Processor for instance. The beast take more than 4x as long, if not longer, than the gaming CPU.

It seems like the real slowdown begins around 32-64 cores on up. The more the cores the longer it takes.

Caustics settings at default. Bucket rendering, Photon mapping.

[Muhammed_Hamed]

Hello,

I'm testing on Threadripper 3970X and 3990X, the slowdown seems to be minimal so far
I'm comparing to 5950X and 7950X, all of the CPUs I have access to locally. I will ask my colleagues to test on EPYC, I think we have one machine at office. We will update you shortly
If we still cannot reproduce, we will ask for a scene.

Best,
Muhammed